Decorative materials used in the applications including building interior materials such as walls, fittings for buildings such as doors, surface materials for furniture, etc., have been usually required to exhibit good surface properties such as abrasion resistance and stain resistance. Hitherto, there has been proposed the decorative sheet having a surface resin layer which is produced, for example, by forming a pattern layer on a substrate made of papers or plastics by a printing method, applying an ionizing radiation-curable resin coating material on the substrate to form a coating resin layer, and then irradiating an electron beam to the coating resin layer for crosslinking and curing the resin composition to form the surface resin layer (for example, refer to Japanese Patent Publication No. 31033/1974). When such a surface resin layer obtained by irradiating an ionizing radiation such as an electron beam to the applied ionizing radiation-curable resin made of monomers, prepolymers, etc., to crosslink and cure the resin is provided as an outermost layer of a decorative sheet, the resultant decorative sheet is excellent in abrasion resistance, stain resistance, etc., due to a high crosslinkability thereof.
However, when irradiating the ionizing radiation for crosslinking the surface resin layer, the conventional decorative materials, for example, those using a substrate made of papers, tend to suffer from cutting or breaking of cellulose molecules of pulps in the paper substrate to thereby generate a carboxyl group or a carbonyl group at a cut or broken end of the molecules. As a result, the paper substrate tends to be deteriorated in strength, resulting in deteriorated processability of the decorative material. Whereas, a plastic substrate also tends to suffer from cutting or breaking of polymer molecular chains upon irradiation of the ionizing radiation, resulting in deterioration in strength thereof. For these reasons, when the decorative sheet is laminated under pressure on the substrate as an adherend such as plywood through an adhesive, the decorative sheet tends to undergo increased tension, resulting in occurrence of split therein due to mechanical vibration, etc.
In particular, when the decorative sheet is laminated on curved portions of the adherend substrate or corner edge portions of such an adherend substrate having a polyhedral column shape by a wrapping process, the decorative sheet tends to suffer from concentrated local stress, resulting in breakage of the decorative sheet. The wrapping method is explained in detail by referring to the conceptual view shown in FIG. 2.
As shown in FIG. 2, a column-shaped substrate B is placed on a transport apparatus 10, and transported in a length direction thereof [MD direction (longitudinal direction: machine direction) of the substrate, i.e., the direction shown by an outlined arrow in FIG. 2]. A wrapping decorative sheet S is fed onto the column-shaped substrate B at a speed synchronized with a transporting speed of the substrate, and laminated on plural side surfaces of the column-shaped substrate by means of a plurality of pressing rollers Ra to Re which are arranged in different directions from each other. The lamination of the wrapping decorative sheet S is stepwise conducted every small area portion thereof in the direction substantially perpendicular or just perpendicular to the longitudinal direction of the column-shaped substrate (CD direction: cross machine direction). The wrapping decorative sheet S is first laminated on a central portion of a side surface of the column-shaped substrate in the width direction thereof by means of the pressing roller Ra, and then successively laminated on adjacent portions of the side surface of the column-shaped substrate by means of the pressing rollers Rb and Rc and further on outer adjacent portions of the side surface of the column-shaped substrate by means of the pressing rollers Rd and Re to cover the desired side surface of the column-shaped substrate with the wrapping decorative sheet. Upon such a wrapping process, since the wrapping decorative sheet is pressed by the pressing rollers Ra to Re under tension applied in the CD direction, the strength thereof, in particular, in the CD direction, is important.
On the other hand, for the purpose of improving a processability of such a decorative material having a surface protective layer obtained by crosslinking and curing an ionizing radiation-curable resin, there has been proposed the decorative material composed of a surface resin layer made of a crosslinked product of the ionizing radiation-curable resin, a paper substrate and a high-modulus resin layer having a specific tensile strength which are laminated on each other from its front-side surface toward its backside surface in this order (refer to claims of Japanese Patent Application Laid-open No. 14490/2002).
Thus, when the high-modulus resin layer is provided on the backside surface of the paper substrate, even though cellulose molecules of pulps of the paper substrate is cut or broken owing to irradiation with the ionizing radiation to thereby cause deterioration in strength of the paper substrate, the deteriorated strength of the paper substrate can be compensated with the high-modulus resin layer on the backside surface, so that the decorative sheet can maintain a suitable strength as a whole. As a result, the resultant decorative sheet can exhibit not only good surface properties such as abrasion resistance owing to the provision of the surface protective layer but also good processability.
However, the above conventional method requires formation of the high-modulus resin layer, resulting in various limitations to production of the decorative materials. Therefore, it has been demanded to provide the method of suppressing deterioration of the substrate itself.